1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device with the use of a crystalline semiconductor thin film. The category of a semiconductor device according to the present invention includes not only devices such as a thin film transistor and a MOS transistor but also an electronic equipment that has a semiconductor circuit consisted of these insulated-gate type semiconductor devices and an electronic equipment such as a personal computer or a digital camera which is provided with an electro-optical display device comprising an active matrix substrate (typically, a liquid crystal display device).
2. Description of the Related Art
A thin film transistor (TFT) is known at present as a semiconductor device using a semiconductor film. The TFT is utilized in various kinds of integrated circuits, especially for a switching device of a matrix circuit in an active matrix type liquid crystal display device. Further, accompanying recent progress in increasing mobility of the TFT, it has become popular to utilize the TFT as a device of a driver circuit for driving the matrix circuit. In order to utilize the TFT for the driver circuit, a semiconductor layer is necessarily a crystalline silicon film in which the mobility is higher than in the amorphous silicon film. This crystalline silicon film is called polycrystalline silicon, polysilicon, microcrystalline silicon, or the like.
A conventionally known method of forming a polycrystalline silicon film includes a method in which a polycrystalline silicon film is directly formed, and a method in which an amorphous silicon film is formed by a CVD method and is subjected to heat treatment at 600 to 1100.degree. C. for 20 to 48 hours to crystallize the amorphous silicon. A polycrystalline silicon film formed by the latter method has larger crystal grains and gives more excellent characteristics to a semiconductor device manufactured from the film.
When a crystalline silicon film is formed on a glass substrate through the latter method, the upper limitation of about 600.degree. C. is put on the process temperature for crystallization, thereby taking a lot of time in crystallizing step. The temperature of 600.degree. C. is close to the lowest temperature for crystallizing silicon, and the temperature equal to or less than 500.degree. C. cannot afford to crystallize silicon in a sufficiently short time period that is paying in terms of industrial production.
To shorten the crystallization period, the use of a quartz substrate having high distortion point and rising a crystallizing temperature to about 1,000.degree. C. are appropriate. However, a quartz substrate is very expensive as compared to a glass substrate, making it difficult to increase the area of the substrate. For instance, Corning 7059 glass that is widely used in active type liquid crystal display device has a glass distortion point of 593.degree. C., and hence the glass substrate suffers shrinkage and deformation when heated at 600.degree. C. or more for several hours. The crystallizing process is therefore required to be lowered in temperature and shortened in time period so that a glass substrate such as Corning 7059 glass can be utilized.
The technique of crystallizing with excimer laser is one of the techniques which enable the process to be lowered in temperature and shortened in time period. An excimer laser light can give, barely putting a substrate under a thermal effect, the semiconductor film an energy comparable to the energy by a thermal annealing of around 1,000.degree. C. in a short period, and can form a semiconductor film of high crystallinity. However, the excimer laser has nonuniform energy distribution on the irradiated surface, with the result that the crystallinity of the obtained crystalline semiconductor film is varied and device characteristics are also varied between TFTs.
Then, the present inventors have disclosed a technique with which the crystallizing temperature is lowered while using a heat treatment in Japanese Patent Application Laid-Open Nos. Hei 6-232059, Hei 7-321339 and others. In the technique of the publications above, a minute quantity of catalytic elements are introduced into an amorphous silicon film to which a heat treatment is subsequently applied to obtain a crystallized silicon film. Used as the elements for promoting the crystallization are elements selected from Ni, Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, Au and Ge, which are invasive elements with respect to silicon.
At the crystallization in the publications above, a heat treatment causes the catalytic elements to be diffused in the amorphous silicon film to advance crystallization of the amorphous silicon film. The employment of the crystallizing technique in the publications above thus makes it possible to form crystalline silicon with a heat treatment of 450 to 600.degree. C. for 4 to 12 hours, which allows the use of a glass substrate.
The crystallization in the publications above, however, has a problem that the catalytic elements are remained in the crystalline silicon film. Remaining catalytic elements impair semiconductor characteristics of the silicon film and damage the stability and the reliability of a device fabricated from the film.
To eliminate this problem, the present applicant has investigated methods of removing (gettering) the catalytic elements from a crystalline silicon film. One of those methods (referred to as the first method) is a heat treatment in an atmosphere containing a halogen element such as chlorine. In this method, the catalytic elements in the film are gasified as halogenate.
Another method (referred to as the second method) among those is a heat treatment subsequent to selective addition of phosphorus into the crystalline silicon film. With the heat treatment, the catalytic elements are diffused into a phosphorus added region and are captured in this region.
However, the first method requires to set the heat treatment temperature to 800.degree. C. or more so that the gettering effect is obtained, and cannot use a glass substrate. On the other hand, the second method has a drawback that the treatment takes ten and several hours, though the heat temperature may be set to 600.degree. C. or less.